1. Field of the Invention
The invention relates to a semiconductor memory, particularly to a nonvolatile memory having functions of electrically rewriting and of verifying data stored therein.
2. Description of the related art
For a semiconductor memory, there are a random access memory capable of freely reading and writing data and a read only memory used specifically for reading data which is written in advance. The read only memory is generally composed of a nonvolatile semiconductor memory. For the read only memory, there is a type capable of electrically rewriting data. Such a memory includes a memory capable of erasing data at a time, namely, by the batch.
The memory capable of electrically rewriting data includes a type having a function of verifying whether a threshold voltage of a memory cell on which data is written is a given voltage or not when given data is written (sometimes called programmed) on a memory cell array composed of a plurality of memory cells incorporated in this memory.
A verification processing using this verification function is executed, for example by a program processing of given data. The verification processing is executed every time given data is written on a memory cell. If it is detected in the verification processing that a memory cell on which data is written does not reach a given threshold, the program processing and verification processing are executed again. Until a memory cell on which data is written reaches a given threshold voltage, the given number of program processing and verification processing are repeatedly executed. If the given number of verification processing is executed and a memory cell on which data is written does not reach a given threshold voltage, program processing is terminated by designating a failure in the program processing.
A demand for representing the maximum value of time needed for a program processing has been recently increased with a warranty of manufacturers of memories. However, the maximum value of time is differentiated depending on the memories and how may times the program processing and verification processing are executed, thus impacting on the termination of the program processing. Accordingly, it scarcely occurs that all memories can execute the preset given number (the maximum number) of program processing and verification processing. Accordingly, it is required that the maximum value of time needed for executing program processing, namely, the time when the preset given number of program processing and verification processing are executed can be measured.
Further, memory has progressed to a high degree of integration, and thus miniaturization of chip size. Accordingly, it is necessary to avoid the sharp increase of the number of circuit configurations in order to measure the maximum value of time needed for executing the program processing. It is further necessary not to impede a normal operation of a memory by a circuit capable of measuring the maximum value of time needed for programming data.
It is another object of the invention to provide a semiconductor memory capable of attaining the above object without sharply increasing the number of circuit configurations.
It is still another object of the invention to provide a semiconductor memory capable of attaining the above object without impeding a normal operation of the memory.